Why is the idea of evolution such a threat for fundamentalists?
It isn't, though they perceive it to be a threat.
My understanding is that they noticed that students were entering the biology class as Christians, but graduating as atheists. So they put 2 and 2 together, came up with 5 as the answer, and blamed evolution.
As far as I can tell, what really happens in biology class is that the students come face to face with the cruelty of nature, and they are unable to reconcile that with what they had been taught in Sunday school.